As is known, an epicyclic transmission comprises a pinion, a ring gear and a plurality of toothed planet wheels, which are interposed between the pinion and the ring gear and are supported by a planet-carrier. A transmission of such a type is capable of transmitting the motion between coaxial shafts rotating at different speeds and is very effective in providing such a function while maintaining contained weight and volumes. Epicyclic transmissions are largely used also in aeronautic engines, for example to transmit the motion to the fan in turbofan engines.
In most applications, the planet-carrier is of stationary type and is coupled to the fixed structure of the engine by means of a flexible element. Under these conditions, the components supported by the planet-carrier (gears of the toothed planet wheels, any bushings, etc.) are lubricated without particular difficulty by means of ducts which are fixed with respect to the engine structure and to the planet-carrier.
Nevertheless, certain applications employ a rotating planet-carrier, for example when the planet-carrier is connected to a (driving or driven) rotating shaft or when there is a need to maintain the same direction of rotation between the pinion and the ring gear. In these cases, the problem occurs of transferring the lubricating oil in an efficient and reliable manner from a stationary part (typically an oil reservoir) to a rotating part, that is the planet-carrier and the components supported thereby.
The solutions actually in use provide one or more pumps which transfer the oil under pressure into an annular cavity about a fixed sleeve with respect to the planet-carrier. The oil radially enters a passage of the sleeve and from here conveyed to the components requiring lubrication. The oil pressure in the annular cavity is guaranteed by seals configured so as to keep a particularly reduced radial clearance between the stationary part and the sleeve. The entity of such a clearance is accurately set by design, so as to maximize the volumetric transfer efficiency of the lubricating oil.
The known oil transfer systems described above are not satisfactory due to the elevated accuracy required in processing and mounting the components which define the aforesaid radial clearance. Furthermore, during assembly there is a non inconsiderable risk of damaging the seals when the sleeve is axially inserted, whereby the correct. sealing is compromised.
Furthermore, the known oil transfer system described above is not able to compensate for the wear and inevitable variations of relative position between the stationary part and the rotating part (caused for example by differences in temperature between the startup conditions and the running conditions). Furthermore, in current epicyclic transmissions for vehicles, the pressure and flow rate values required for the lubrication and the peripheral speed values of the planet-carrier are relatively high and therefore may compromise the desired sealing, especially when the components are worn.